1. Field of the Invention
This invention is in the field of membrane separations of electrophoretic paints and particularly relates to the cleaning of a membrane which has been fouled by a cataphoretic paint containing lead.
2. Brief Description of the Prior Art
Cationic electrodeposition of resinous materials onto metallic articles has become of great commercial significance in the appliance and automotive industries. Cationic electrodeposition provides coatings with outstanding corrosion resistance, usually much better than that provided by anionic electrodeposition. In anionic electrodeposition, the acidic resin is electrodeposited onto a metallic article serving as an anode and any residual acidity in the resin reduces its corrosion resistance. Further, metal ions from the anode are trapped within the film and they also adversely affect the corrosion resistance and color of the resultant coating. Cationic electrodeposition avoids these problems because the depositing resin is basic and there is no metal ion dissolution at the cathode.
A major problem in practicing electrodeposition on a continuous basis is in removing impurities and maintaining the electrodeposition bath properties. The solubilized electrodepositable resin is a polyelectrolyte and when the vehicle resin is coated upon the electrode, there remains in solution a counter ion which is used to solubilize the resin. In addition, as the electrodeposition proceeds, chemicals from various sources tend to accumulate in the electrodeposition bath. Contaminants such as phosphates and chromates are carried into the bath with the articles to be coated and the paint itself can also introduce contaminants.
Ultrafiltration has been found to be a particularly effective means of removing impurities and maintaining the properties of the electrodeposition bath. Ultrafiltration removes from the bath low molecular weight impurities such as counter ion, phosphates, chromates and low molecular weight resinous fragments while retaining the desirable high molecular weight resinous materials and pigments. Besides helping to maintain the properties of the electrodeposition bath, ultrafiltration is also a means of generating rinse water for removing dragout from the freshly electrocoated articles.
With anionic electrodeposition paints, ultrafiltration can be operated continuously for many months at a time without the membrane fouling or becoming clogged. However, it has been found that with cationic electrodeposition paints containing lead such as lead pigments, fouling or clogging of the membrane is a much more serious problem. Although the cause of the problem is not completely understood, it is believed that the acidic pH of the cataphoretic electrodeposition baths solubilize some of the lead pigment. Many of the chemicals used to pretreat the articles being coated, that is, phosphates and chromates which are carried into the bath with the articles, react with the solubilized lead and precipitate colloidal lead complexes which foul the membrane. Whatever the reason, experience has shown that typical commercially available ultrafiltration membranes which are used on a continuous basis for the treatment of anionic paints containing lead foul and become plugged much more quickly when used with cationic paints containing lead. Membrane fouling is a serious problem in that it not only detrimentally affects the properties of the electrodeposition bath, but also restricts the generation of needed rinse water.